The present invention relates to an improved electrostatic copying machine or apparatus. In such an apparatus a primary corona charging unit applies a uniform electrostatic charge to a rotating photoconductive drum, belt or the like. A light image of an original document is radiated onto the drum to produce an electrostatic image through localized photoconduction. A developing unit applies toner to develop the electrostatic image into a toner image. The toner image is transferred and fixed to a copy sheet to provide a permanent copy. A transfer corona charger urges transfer of the toner image from the drum to the copy sheet whereas a separation corona charger facilitates separation of the copy sheet from the drum. A discharge corona charger typically energized with alternating voltage discharges the drum prior to recharging by the primary charger.
A problem has existed in such an apparatus in that the density of a first copy is lower than the density of subsequent copies. It has been determined that this problem is due to the fact that the electrostatic charge on the surface of the drum varies at different locations due to the status of the various corona chargers at the time the apparatus was de-energized at the termination of a previous copying operation.
Various means have been proposed to overcome this problem. One such expedient has been to increase the A.C. potential applied to the discharging corona charger. While this has succeeded in reducing the difference between the densities of the first and subsequent copies, it has introduced another problem in that a space charge of the same polarity as the primary charge injected from a conductive substrate or core of the drum into a photoconductive layer thereof is formed at the interface of the core and photoconductive layer. This space charge layer has a very detrimental effect on the image quality. Such a high A.C. potential is accompanied by a substantial increase in electrical power consumption and increases the electrical shock hazzard.
Another expedient has been to illuminate the drum with light in addition to or instead of applying the A.C. corona discharge. This prior art method, while somewhat reducing the difference in density between the first copy and subsequent copies does not completely eliminate said difference and accelerates the fatigue of the photoconductive drum.